SINGAPORE (BLOOMBERG) - S&P Global Ratings said that several sectors in Singapore's bond market are vulnerable to "rising financial strain" after a number of recent defaults.

Indonesia-based mobile phone distributor and retailer Trikomsel Oke Tbk defaulted in November 2015, followed by Hong Kong-based industrial fishing company Pacific Andes Resources Development in January. Most recently, Swiber Holdings, an offshore oil and gas services group, roiled the local bond market when it defaulted on its local-currency notes last month, becoming the third entity to renege on payments since November.

The oil services sector is the most vulnerable, while agriculture and commodities companies and some real estate companies may also face stresses ahead, the ratings firm said in a statement on Friday.

"In our view, smaller developers are more vulnerable than their larger peers to near-term market volatility often due to weak liquidity positions and sometimes unsustainable leverage levels," said S&P analyst Bertrand Jabouley in the statement. For commodity-related companies, liquidity remains fragile on declining earnings, he said.

S&P estimated that listed entities in Singapore have about S$60 billion in bonds outstanding as of Sept 15.

Excluding all bonds issued by government-related entities, the amount is about S$53 billion, according to the ratings company. Real estate firms account for a little over half of the total, S&P said.